The invention is directed generally to the fuel cell arts and more particularly concerns molten carbonate electrolyte fuel cells, and still more particularly concerns a novel creepage barrier for a molten carbonate electrolyte fuel cell.
A fuel cell is an energy conversion device which continuously and directly converts chemical energy of a fuel into electrical energy by an electrochemical process. Generally speaking, a fuel cell is analagous to a familiar dry cell battery, in that it comprises a pair of electrodes placed in contact with a liquid or solid electrolyte. However, unlike the dry cell battery, the fuel cell uses fuel such as hydrogen which is introduced at one electrode. An oxidant such as oxygen from the surrounding air enters at the other electrode. The fuel is oxidized in an electrochemical reaction which takes place at the interface between the electrodes and the electrolyte. The oxidation of the fuel releases a flow of electrons between the anode and the cathode. Hence, the anode and cathode may be coupled to an external electrical circuit to produce a flow of electrical current therethrough. Unlike the dry cell battery, a fuel cell does not, at least in theory, run down or require recharging; rather, it will operate as long as the fuel and oxidant continue to be supplied to the electrodes.
A molten carbonate fuel cell is one which utilizes a molten carbonate substance as the electrolyte. Such a molten carbonate electrolyte is solid at room temperature and becomes a molten liquid at operating temperatures which may range between 500 degrees C. and 750 degrees C. Such fuel cells are shown for example in U.S. Pat. Nos. 4,009,321 and 4,079,171. Such molten carbonate electrolytes may comprise alkali metal carbonate compositions, such as lithium, sodium or potassium carbonates. This electrolyte is preferably provided in the form of a substantially inert matrix sandwiched between an anode electrode and a cathode electrode. One such matrix is shown and described in U.S. Pat. No. 4,411,968, which also sets forth additional suitable electrolyte materials.
A sing1e fuel cell of the type described produces a relatively low voltage of on the order of 1.0 volts DC. However, such fuel cells advantageously exhibit a relatively high current density capability. Hence, the power density (Watts per unit area) which may be generated by such fuel cells is relatively high. According1y, higher voltages are obtained by placing individual fuel cells in a series configuration in what is generally termed a "fuel cell stack". In such a configuration, the individual cells are quite literally stacked one on top of another to obtain a desired voltage from the series circuit configuration thus obtained.
One significant problem which has arisen with respect to such fuel cell stacks is the loss of electrolyte material therefrom. This electrolyte loss is believed to result from a number of effects, a significant one of which is termed electrolyte creepage. By creepage is meant the tendency of the electrolyte to creep or flow in films along the surfaces of the fuel cell stack and toward the negative or anode end thereof. A continuous loss of electrolyte in this fashion results in gradual corresponding withdrawal of electrolyte from the pores of the matrix. This in turn is believed to result in a formulation of gas pockets in the matrix which cause the internal resistance of the fuel cells to rise, thereby causing a decline in the voltage produced across the fuel cell. As the loss of electrolyte becomes progressively greater, the cell resistance thus dramatically increases, causing a rapid and non-recoverable drop in the cell voltage.